Such lemmas are systematically studied and applied in [1]}. Their
importance is underlined in the article [2]}, in
which the authors establish a priori bounds for elliptic
equations of second order with bounded measurable coefficents. Nowadays they
form a standard
tool for the study of various questions of nonlinear partial differential
equations of second order, cf. [3]} and [4]}. Note that the
property
formulated in lem:growth is also referred to as expansion
of positivity which describes the corresponding property for \(1-u\) .
